Background: During the early 2000s then-President Mbeki publicly questioned AIDS science and suggested antiretroviral therapy is harmful. It has been suggested that this contributed to public confusion and encouraged risky sexual behaviour. This study investigates the prevalence and determinants of conspiracy beliefs (about the origins of AIDS) and denialist beliefs (about its existence and causes), its determinants and effects on sexual behaviour.Methods: The analysis is based on the 2009 wave of the Cape Area Panel/cohort Study, a survey of 3149 respondents aged 19-35 from all races and income groups in Cape Town, South Africa. Indices for conspiracy and denialist beliefs respectively were constructed from a set of attitudinal questions. By means of a series of logistic multivariate regressions, we constructed a number of models to explain variation in these beliefs, incorporating demographic, socio-economic and attitudinal correlates. A second set of models investigate the relationship with risky sexual behaviour. Finally, we present follow-up qualitative data collected during in-depth semi-structured interviews with a subset of respondents on conspiracy and denialist beliefs.Results: Conspiracy and denialist beliefs are prevalent among a significant minority of respondents and both are associated with risky sexual behaviour. However, conspiracy and denialist beliefs tap into different oppositional transcripts on AIDS and resonate differently for men and women. AIDS denialist beliefs also takes on a broader cross-class character. Importantly, the odds of holding conspiracy beliefs and denialist beliefs were significantly and substantially higher if the respondent trusted Mbeki´s Health Minister more than the minister who replaced her. Awareness of the Treatment Action Campaign reduced the likelihood of holding denialist beliefs (though this may be proxying for social environment).Conclusions: These results point to the importance of political leadership in shaping attitudes and behaviours with regard to AIDS pathogenesis, prevention and treatment. The effects of political AIDS denialism in South Africa linger on.